The joining of similar and dissimilar metals is a technical topic that is of great practical importance and can be quite problematic. This is an issue that is important in a variety of industries such as the automotive, aerospace, medical, micro-electronics, and food and beverage industries. Broadly there are two ways that joints can be formed between metals: mechanical interlocks or metallurgical bonds. There are a variety of subcategories of each of these.
Mechanical interlocks can be formed along linear features by variants on folding metal across itself to create folded seams. Examples of this kind of joining can be found in the joints that hold can ends on can bodies and automobile closure panels such as doors where an outer panel is interlocked with an inner panel with a fold known as a hem. Mechanical interlocks can also be used at a discrete location. This kind of feature is known as a clinch or clinch-lock and is produced by deforming two sheets of metal into two interlocking cup-like or reentrant features. This type of interlock is almost universally used to hold the tab onto the end of a beverage can.
Joining between metal components is often accomplished by welding, where the defining characteristic is that the two metal surfaces become adhered to one another. This is typically done where both metals are melted (by heat provided in one of numerous ways) until the two metals become mixed (sometimes with a filler metal added) and upon solidification a joint is formed. Fusion welding has two very significant drawbacks. First, it is limited to alloys that make compatible pairs when mixed. Many alloy systems will form brittle intermetallics when mixed and this provides poor mechanical performance. Second, the heat that is deposited in the weld zone usually dramatically softens the material adjacent to the weld and the melted and solidified material itself may be quite weak or brittle. As a result, the metals that make up welded structures typically have quite low strength in the region that makes up the weld. Other related alternatives to fusion welding also exist. Solid state welding can be accomplished by variants of resistance or of spot welding, high velocity collision welding (as is usually accomplished using explosives) and ultrasonic welding. Also, interlayers can be used in methods such as brazing and soldering. These techniques can all have significant disadvantages for when considering specific applications.
It is well known that if two relatively clean metal surfaces collide with an appropriate velocity (usually over 200 m/s and typically in the range of from about 150 to 500 m/s) and the impact angle is within a proper range (usually about 15 degrees and typically in the range of from about 5 to about 25 degrees) the two surfaces may adhere forming a metallurgical joint. This technology is well developed and fairly well understood and practiced in explosive welding and electromagnetic welding. In both these cases a sheet known as a flyer plate is usually driven at high speed either by the explosive or electromagnetic force and strikes a sheet that is often thicker referred to as a target. Upon striking there is little overall increase in temperature and the material is often hardened by the local plastic strain of impact. Similar or dissimilar metals can be joined in this manner as there is not sufficient time or temperature for the formation of brittle intermetallics. Metallurgical joints formed in this manner can have the strength of the parent material.
However, explosion welding has some disadvantages, not the least of which are the safety and containment issues associated with the process. Additionally, it is very difficult to couple explosives or magnetic pulse energy to small structures (on the order of millimeters or smaller). However, collision processes also have some tremendous advantages; most notably the fact that virtually any metal combination can be joined using this process. Because the heat required for joining is localized at the interface and quickly dissipated by the materials that are being joined, explosion welds typically do not form large or interconnected regions of embrittling intermetallic compounds (even in complex systems) and the residual stresses generated are essentially nil. For these reasons, the potential for joining materials using high velocity collisions that do not require the use of explosives offers the potential for joining otherwise difficult (or impossible) to join materials. These include many of the advanced structural materials based on intermetallic compounds, composites, nano-structured materials and metallic glasses.